1. Field of the Invention
The present invention relates to a discharging control circuit, and more particularly, to a control circuit for electronic discharging using energy storing device to reduce the residual image phenomenon of an LCD.
2. Description of Related Art
In a thin film transistor fabricating process, the major steps include coating photoresist (PR), exposure, development, film deposition and etching. Wherein the step of coating PR includes loading a glass substrate to a spinner, yet this method results in thinner PR around central area than that around the peripheral of the glass substrate, therefore during the etching step, for example, a back channel etching (BCE), the etching rate around the central area of the glass substrate is faster than that around the peripheral. Thus, an off-leakage current of a TFT is lower. Accordingly, the off-leakage current of a TFT varies according to its location on the substrate, thus the speed of discharging the pixel capacitor varies depending on the location on the substrate. Therefore, pixel capacitors originally located around central area of the glass substrate require longer time to discharge.
Referring to FIG. 1, a schematic diagram of a conventional LCD panel fabricated on a glass substrate using TFT fabrication technology is shown. As shown, the LCD panel 100 is divided into four blocks, the structure of the upper right block 102 is described as an example. An area referenced by numeral 110, hereinafter area 110, indicates a rough border between high and low channel turn-off leakage current of a pixel transistor. When the power is off, since it takes longer time for discharging the transistors located in the area 110, it takes longer time for the image to diminish from the corresponding pixels. From a user's point of view, a residual image phenomenon resembling an ebb tide is investigated, and a dynamic subsidence direction is shown as the arrow in block 102 in FIG. 1.
Moreover, the thin film transistor fabricating process for an LCD usually results in a difference from 105 to 106 times between a turn-on current Ion and a turn-off current Ioff corresponding to a gate of the transistor. For example, when a gate potential of a turned-on transistor is 24 V, Ion is in the order of μA, whereas when the gate potential of a turned-off transistor is −6V, Ioff is in the order of pA. When the gate of the transistor is in off state, since Ioff is far smaller than Ion, for a high-resolution display panel, it is more often to observe residual image phenomenon. In other words, since film thickness as well as capacitance of transistors are different, when the power supply is turned off, the time required for discharging is different and also the time required for liquid crystals thereof to twist back to original position is different. Therefore, residual image phenomenon is observed on the panel. Meanwhile, the TFTs are switched to an off state, and pixel capacitors are discharged merely via data scanning line, and the Ioff is in the order of pA as mentioned above, slow discharging is relatively obvious and uneven with respect to persistence in the eyes of a viewer. That is, residual image phenomenon, resembling the tide, is observed. This specific phenomenon cannot be improved by merely rising Ioff, since rising Ioff in the specifications of a TFT would deficit other characteristics of an image, e.g. flickering phenomenon. Therefore, in order to eliminate residual image phenomenon, one of the efficient solutions is to elaborate on circuit design thereof.
Referring to FIG. 2A, a schematic diagram of a circuit 200 pulling gate potential of a transistor to ground level GND in power off-state is illustrated. The circuit pulls the gate potential to ground level more rapidly using an energy storage device. Referring to FIGS. 2B and 2C, differential temporal response for gate potential using different circuit designs is observed. However, along with the mechanism according to conventional art, gate potential is rapidly pulled to ground level GND only when TFT is switched off. If TFT turned-on threshold voltage does not effectively switch on the gate of the TFT, a tide phenomenon is still observed. Therefore, it is necessary to have a novel integrated circuit for discharging pixel capacitance as well as pulling down gate potential even when power supply is off, such that the tide phenomenon of an LCD panel can be reduced.